The present invention relates to a heat-recoverable article for bonding to an object such as a pipe, for example to seal the object against ingress of moisture or to protect against physical damage, or to make a joint between the object and another object.
A heat-recoverable article is one whose dimensional configuration can be made to change substantially when subjected to heat treatment. A heat-recoverable article will usually recover towards an original shape from which it has previously been deformed but the term as used herein also includes articles which adopt a new configuration when subjected to heat treatment, even if it has not previously been deformed.
Heat-recoverable articles may be used in the sealing of pipe joints, the repair of polymeric coatings on pipes and similar structures, the protection from chemical and physical attack of pipes and other structures, often metallic They may also be used to seal cable splices.
When a joint is made between two lengths of thermally insulated steel pipe, as used for example in a district heating system, it is necessary to remove the insulation from the end regions of each length of pipe in order that the insulation is not burnt when the pipes are joined, for example by welding. After the weld has been completed, the uninsulated portion of the pipe in the region of the joint must be reinsulated in order to minimise heat loss. It is important that the insulation in the region of the joint is sealed against moisture ingress which might otherwise cause degradation of the insulation, not only in the joint region but possibly also along the pipes to some distance from the joint, and cause also corrosion of the pipes.
Severe physical constraints are placed upon the seal of the insulation at a joint between insulated pipes in a district heating system. As a result of frequent and wide temperature cycling, the pipe can be subject to expansion and contraction cycles and to longitudinal and transverse movements. For example, it has been determined that forces of up to 10 N.mm.sup.-2 can be exerted on the outer protective jacket of an insulated pipe as a result of temperature cycles experienced by a pipe in winter before it is buried. The seal must be able to withstand these cycles and movements without breaking. Furthermore, it is preferred that the seal be able to withstand the pressures exerted when the cavity around the uninsulated portion of pipe is reinsulated by foaming in situ.
One approach that has met with considerable success is the subject of GB No.-A-2108625. In one aspect, it involves creating a flexible seal, between the insulation surrounding each pipe and a rigid casing which spans the uninsulated portion of pipe, by means of a heat-recoverable polymeric sleeve coated with a sealant. To prevent movement of the sealant coated sleeve of GB No.-A-2108625 relative to the pipe, the sleeve is provided with adhesive material at specified regions to bond the sleeve to the pipe insulation and to the casing.
The flexible seal approach described above has been found to be satisfactory in the majority of applications. There is however a demand for a seal for the insulation at a pipe joint which is rigid and which is sufficiently strong to withstand the forces encountered during installation and in SO use without stretching or flexing to any significant extent, or breaking. One example of a rigid seal system for an insulated pipe joint is described in EP-A-No. 138864.
In order to achieve the strength required of a rigid seal, it would be necessary for the heat-recoverable sleeve to be relatively thick, having a thickness of for example at least 2 mm, the actual thickness depending to some extent on the size of the pipes to be connected. When heating a sleeve of such thickness by conventional means such as a gas torch or hot air gun (as disclosed in EP-A-No. 138864), there is a danger of burning the outside of the sleeve before the inside of the sleeve has been heated sufficiently for it to recover, particularly in the case of sleeves with a thickness of at least 5 mm. This can lead to failure of the seal by splitting of the sleeve.
We have now devised a heat-recoverable article, comprising an inbuilt laminar heating element for heating the article to effect recovery, which element can be deformed as the article recovers.